My interpretation of the "DO NOT MESS WITH TIME" incident was that you can try setting things up so that temporal consistency implies the result you want, but actually ruling out every other possibility whatever, including vast classes of outcomes you never even imagined, is next to impossible, at least for an 11-year-old boy, however smart. It hadn't even occurred to him that there was a problem in his reasoning when he tried the experiment that gave him "the scariest result ever in the history of science". Temporal consistency in the presence of time loops is another blind idiot god, far more swift and powerful than evolution.

I think you summed that up perfectly. I would have liked to see a little more of that explanation in the Fan Fic. It would make the story feel more natural and also be a perfect excuse to include the 'blind idiot god' kind of message.

Anyway, he still has the Time-Turner. All that stands between him and it is a magical lock. How difficult can that be for him to get around?

Without Hermione backing him up? I think he'd struggle. Harry just isn't that smart! He really should be spending more time sweet talking her. Well intentioned genius with ambition that doesn't involve gaining power... just the kind of ally Harry needs.

No, actually what stands between him and it is the author's necessity not to unbalance the plot by giving him a get-out-of-jail-free card.

Uh huh. It's even worse than if Harry had actually gone and made himself rich on day 1!